English

Alternative spellings

Pronunciation

This vacuum means "absent of matter", "An
empty Area or space"; for the cleaning appliance, see vacuum
cleaner.

A vacuum is a volume of space that is essentially empty of
matter, such that its
gaseous pressure is
much less than atmospheric
pressure. The word comes from the Latin term for "empty," but
in reality, no volume of space can ever be perfectly empty. A
perfect vacuum with a gaseous pressure of absolute zero is a
philosophical concept that is never observed in practice. Physicists often
discuss ideal test results that would occur in a perfect vacuum,
which they simply call "vacuum" or "free space" in
this context, and use the term partial vacuum to refer to real
vacuum. The Latin term in vacuo is also used to describe an object
as being in what would otherwise be a vacuum.

The quality of a vacuum refers
to how closely it approaches a perfect vacuum. The residual gas
pressure is the primary
indicator of quality, and is most commonly measured in units called
torr, even in metric contexts. Lower
pressures indicate higher quality, although other variables must
also be taken into account. Quantum
theory sets limits for the best possible quality of vacuum,
predicting that no volume of space can be perfectly empty. Outer space
is a natural high quality vacuum, mostly of much higher quality
than can be created artificially with current technology. Low
quality artificial vacuums have been used for suction for many
years.

Outer space

Outer space
has very low density and pressure, and is the closest physical
approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal
gravitational trajectories. But no vacuum is truly perfect, not
even in interstellar space where there are still a few hydrogen
atoms per cubic centimeter. The deep vacuum of space could make it
an attractive environment for certain industrial processes, for
instance those that require ultraclean surfaces; however, it is
much less costly to create an equivalent vacuum on Earth than to
leave the Earth's gravity
well.

Stars, planets and moons keep
their atmospheres by
gravitational attraction, and as such, atmospheres have no clearly
delineated boundary: the density of atmospheric gas simply
decreases with distance from the object. The Earth's atmospheric
pressure drops to about 1 Pa (10-3 Torr) at 100 km of
altitude, the Kármán
line which is a common definition of the boundary with outer
space. Beyond this line, isotropic gas pressure rapidly becomes
insignificant when compared to radiation
pressure from the sun
and the dynamic
pressure of the solar wind, so
the definition of pressure becomes difficult to interpret. The
thermosphere in
this range has large gradients of pressure, temperature and
composition, and varies greatly due to space
weather. Astrophysicists prefer to use number
density to describe these environments, in units of particles
per cubic centimetre.

But although it meets the
definition of outer space, the atmospheric density within the first
few hundred kilometers above the Kármán line is still sufficient to
produce significant drag on
satellites. Most
artificial satellites operate in this region called low earth
orbit and must fire their engines every few days to maintain
orbit. The drag here is low enough that it could theoretically be
overcome by radiation pressure on solar sails, a
proposed propulsion system for interplanetary
travel. Planets are too massive for their trajectories to be
affected by these forces, although their atmospheres are eroded by
the solar winds.

Effects on humans and
animals

Humans and animals exposed to
vacuum will lose consciousness after a few
seconds and die of hypoxia within minutes, but the
symptoms are not nearly as graphic as commonly shown in pop
culture. Blood and other body
fluids do boil when their pressure drops below 6.3 kPa, (47 Torr,)
the vapour
pressure of water at body temperature. This condition is called
ebullism. The steam may
bloat the body to twice its normal size and slow circulation, but
tissues are elastic and porous enough to prevent rupture. Ebullism
is slowed by the pressure containment of blood vessels, so some
blood remains liquid. Swelling and ebullism can be restrained by
containment in a flight suit.
Shuttle
astronauts wear a fitted elastic garment called the Crew Altitude
Protection Suit (CAPS) which prevents ebullism at pressures as low
as 2 kPa (15 Torr). Rapid evaporative cooling of the skin will
create frost, particularly in the mouth, but this is not a
significant hazard.

Animal experiments show that
rapid and complete recovery is the norm for exposures shorter than
90 seconds, while longer full-body exposures are fatal and
resuscitation has never been successful. There is only a limited
amount of data available from human accidents, but it is consistent
with animal data. Limbs may be exposed for much longer if breathing
is not impaired. Injuries caused by rapid decompression are called
barotrauma. A
pressure drop as small as 100 Torr, (13 kPa,) which produces no
symptoms if it is gradual, may be fatal if occurs suddenly. The
philosopherAl-Farabi (872 -
950 CE) appears to
have carried out the first experiments concerning the existence of
vacuum, in which he investigated handheld plungers in water. He
concluded that air's volume can expand to fill available space, and
he suggested that the concept of perfect vacuum was
incoherent.

In the Middle Ages,
the Catholic Church held the idea of a vacuum to be immoral or even
heretical. The absence of anything implied the absence of God, and harkened back
to the void prior to the creation story in the book of Genesis. Medieval
thought
experiments into the idea of a vacuum considered whether a
vacuum was present, if only for an instant, between two flat plates
when they were rapidly separated. There was much discussion of
whether the air moved in quickly enough as the plates were
separated, or, as Walter
Burley postulated, whether a 'celestial agent' prevented the
vacuum arising. The commonly held view that nature abhorred a
vacuum was called horror
vacui. This speculation was shut down by the 1277 Paris
condemnations of BishopEtienne
Tempier, which required there to be no restrictions on the
powers of God, which led to the conclusion that God could create a
vacuum if he so wished. René Descartes also argued against the
existence of a vacuum, arguing along the following lines:“Space is
identical with extension, but extension is connected with bodies;
thus there is no space without bodies and hence no empty space
(vacuum)”. In spite of this, opposition to the idea of a vacuum
existing in nature continued into the Scientific
Revolution, with scholars such as Paolo Casati
taking an anti-vacuist position. Jean Buridan
reported in the 14th century that teams of ten horses could not
pull open bellows when
the port was sealed, apparently because of horror vacui. Berti's
barometer produced a vacuum above the water column, but he could
not explain it. The breakthrough was made by Evangelista
Torricelli in 1643. Building upon Galileo's notes, he built the
first mercurybarometer and wrote a
convincing argument that the space at the top was a vacuum. The
height of the column was then limited to the maximum weight that
atmospheric pressure could support. Some people believe that
although Torricelli's experiment was crucial, it was Blaise
Pascal's experiments that proved the top space really contained
vacuum.

In 1654, Otto von
Guericke invented the first vacuum pump and conducted his
famous Magdeburg
hemispheres experiment, showing that teams of horses could not
separate two hemispheres from which the air had been evacuated.
Robert
Boyle improved Guericke's design and conducted experiments on
the properties of vacuum. Robert Hooke
also helped Boyle produce an air pump which helped to produce the
vacuum. The study of vacuum then lapsed until 1855, when Heinrich
Geissler invented the mercury displacement pump and achieved a
record vacuum of about 10 Pa (0.1 Torr). A number of
electrical properties become observable at this vacuum level, and
this renewed interest in vacuum. This, in turn, led to the
development of the vacuum
tube.

While outer space has been
likened to a vacuum, early theories of the nature of light relied upon the existence of
an invisible, aetherial medium which would convey waves of light
(Isaac
Newton relied on this idea to explain refraction and radiated
heat). This evolved into the luminiferous
aether of the 19th century, but the idea was known to have
significant shortcomings - specifically that if the Earth were
moving through a material medium, the medium would have to be both
extremely tenuous (because the Earth is not detectably slowed in
its orbit), and extremely rigid (because vibrations propagate so
rapidly). An 1891 article by
William
Crookes noted: "the [freeing of] occluded gases into the vacuum
of space". Even up until 1912, astronomerHenry
Pickering commented: "While the interstellar absorbing medium
may be simply the ether, [it] is characteristic of a gas, and free
gaseous molecules are certainly there".

In 1887, the Michelson-Morley
experiment, using an interferometer to attempt
to detect the change in the speed of
light caused by the Earth moving with
respect to the aether, was a famous null result, showing that there
really was no static, pervasive medium throughout space and through
which the Earth moved as though through a wind. While there is
therefore no aether, and no such entity is required for the
propagation of light, space between the stars is not completely
empty. Besides the various particles which comprise cosmic
radiation, there is a cosmic
background of photonic radiation (light),
including the thermal background at about 2.7 K, seen as a relic of
the Big
Bang. None of these findings affect the outcome of the
Michelson-Morley experiment to any significant degree.

Einstein argued that physical
objects are not located in space, but rather have a spatial extent.
Seen this way, the concept of empty space loses its meaning.
Rather, space is an abstraction, based on the relationships between
local objects. Nevertheless, the
general theory of relativity admits a pervasive gravitational
field, which, in Einstein's words, may be regarded as an "aether",
with properties varying from one location to another. One must take
care, though, to not ascribe to it material properties such as
velocity and so on.

In 1930, Paul Dirac
proposed a model of vacuum as an infinite sea of particles
possessing negative energy, called the Dirac sea. This
theory helped refine the predictions of his earlier formulated
Dirac
equation, and successfully predicted the existence of the
positron, discovered
two years later in 1932. Despite this
early success, the idea was soon abandoned in favour of the more
elegant quantum
field theory.

The development of quantum
mechanics has complicated the modern interpretation of vacuum
by requiring indeterminacy.
Niels
Bohr and Werner
Heisenberg's uncertainty
principle and Copenhagen
interpretation, formulated in 1927, predict a
fundamental uncertainty in the instantaneous measurability of the
position and momentum
of any particle, and which, not unlike the gravitational field,
questions the emptiness of space between particles. In the late
20th century, this principle was understood to also predict a
fundamental uncertainty in the number of particles in a region of
space, leading to predictions of virtual
particles arising spontaneously out of the void. In other
words, there is a lower bound on the vacuum, dictated by the lowest
possible energy state of the quantized fields in any region of
space.

Quantum-mechanical
definition

detail vacuum state
In quantum mechanics, the vacuum is defined as the state (i.e.
solution to the equations of the theory) with the lowest energy. To
first approximation, this is simply a state with no particles,
hence the name.

Even an ideal vacuum, thought
of as the complete absence of anything, will not in practice remain
empty. Consider a vacuum chamber that has been completely
evacuated, so that the (classical) particle concentration is zero.
The walls of the chamber will emit light in the form of black
body radiation. This light carries momentum, so the vacuum does
have a radiation pressure. This limitation applies even to the
vacuum of interstellar space. Even if a region of space contains no
particles, the
Cosmic Microwave Background fills the entire universe with
black body radiation.

An ideal vacuum cannot exist
even inside of a molecule. Each atom in the molecule exists as a
probability function of space, which has a certain non-zero value
everywhere in a given volume. Thus, even "between" the atoms there
is a certain probability of finding a particle, so the space cannot
be said to be a vacuum.

Outgassing

Evaporation and
sublimation
into a vacuum is called outgassing. All materials,
solid or liquid, have a small vapour
pressure, and their outgassing becomes important when the
vacuum pressure falls below this vapour pressure. In man-made
systems, outgassing has the same effect as a leak and can limit the
achievable vacuum. Outgassing products may condense on nearby
colder surfaces, which can be troublesome if they obscure optical
instruments or react with other materials. This is of great concern
to space missions, where an obscured telescope or solar cell can
ruin an expensive mission.

The most prevalent outgassing
product in man-made vacuum systems is water absorbed by chamber
materials. It can be reduced by desiccating or baking the chamber,
and removing absorbent materials. Outgassed water can condense in
the oil of rotary
vane pumps and reduce their net speed drastically if gas
ballasting is not used. High vacuum systems must be clean and free
of organic matter to minimize outgassing.

Ultra-high vacuum systems are
usually baked, preferably under vacuum, to temporarily raise the
vapour pressure of all outgassing materials and boil them off. Once
the bulk of the outgassing materials are boiled off and evacuated,
the system may be cooled to lower vapour pressures and minimize
residual outgassing during actual operation. Some systems are
cooled well below room temperature by liquid
nitrogen to shut down residual outgassing and simultaneously
cryopump the
system.

Quality

The
quality of a vacuum is indicated by the amount of matter remaining
in the system, so that a high quality vacuum is one with very
little matter left in it. Vacuum is primarily measured by its
absolute
pressure, but a complete characterization requires further
parameters, such as temperature and chemical
composition. One of the most important parameters is the mean free
path (MFP) of residual gases, which indicates the average
distance that molecules will travel between collisions with each
other. As the gas density decreases, the MFP increases, and when
the MFP is longer than the chamber, pump, spacecraft, or other
objects present, the continuum assumptions of fluid
mechanics do not apply. This vacuum state is called high
vacuum, and the study of fluid flows in this regime is called
particle
gas dynamics. The MFP of air at atmospheric pressure is very
short, 70 nm, but at
100 mPa (~1×10-3
Torr) the MFP of room temperature air is roughly 100 mm, which is
on the order of everyday objects such as vacuum tubes.
The Crookes
radiometer turns when the MFP is larger than the size of the
vanes.

Vacuum quality is subdivided
into ranges according to the technology required to achieve it or
measure it. These ranges do not have universally agreed
definitions, but a typical distribution is as follows:

Low vacuum, also called rough vacuum or
coarse vacuum, is vacuum that can be achieved or measured with
rudimentary equipment such as a vacuum
cleaner and a liquid column manometer.

Medium vacuum is vacuum that can be achieved
with a single pump, but is too low to measure with a liquid or
mechanical manometer. It can be measured with a McLeod gauge,
thermal gauge or a capacitive gauge.

High vacuum is vacuum where the MFP of
residual gases is longer than the size of the chamber or of the
object under test. High vacuum usually requires multi-stage pumping
and ion gauge measurement. Some texts differentiate between high
vacuum and very high vacuum.

Ultra high vacuum requires baking the
chamber to remove trace gases, and other special procedures.
British and German standards define ultra high vacuum as pressures
below 10-6 Pa (10-8 Torr).

Deep space is generally much more empty than
any artificial vacuum that we can create. It may or may not meet
the definition of high vacuum above, depending on what region of
space and astronomical bodies are being considered. For example,
the MFP of interplanetary space is smaller than the size of the
solar system, but larger than small planets and moons. As a result,
solar winds exhibit continuum flow on the scale of the solar
system, but must be considered as a bombardment of particles with
respect to the Earth and Moon.

Perfect vacuum is an ideal state that cannot
be obtained in a laboratory, nor can it be
found in outer space.